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(11) | EP 0 505 750 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Ceramic anode for oxygen evolution, method of production and use of the same |
| (57) The anode of the invention is particularly suitable for oxygen evolution from acid
solutions containing fluorides or fluorocomplex anions, used for deposition of metals. The anode is of the ceramic type, consisting of tin dioxide comprising additives to promote sinterization and increase the electrical conductivity. In a preferred embodiment of the invention the ceramic anode comprises an electrocatalytic layer for oxygen evolution comprising oxides of manganese, cerium and praseodymium, suitably doped. In a particularly preferred embodiment of the present invention said anode comprises a further external layer of zirconyl phosphate. |
[0001] Electrolytes containing anionic fluorocomplexes are commonly used in conventional technologies for the electrolytic recovery of metals, such as lead, tin, chromium. In the specific case of lead recovery from batteries scraps, the scraps are leached with acid solutions containing tetrafluoroborates BF₄⁻ and hexafluorosilicates SiF₆=. The electrolysis of these solutions produces lead as a solid deposit: therefore the electrolytic cells are diaphragmless and have a very simple design. However, this advantage has been so far counterbalanced by the scarce resistance of the substrates to the aggressive action of anionic fluorocomplexes on the anodes whereat oxygen is evolved. Further a parasitic reaction may take place with formation of lead diode which subtracts lead to the galvanic deposition of the metal, thus reducing the overall efficiency of the system.
[0002] Upon carefully considering the prior art teachings found for example in U.S. 3,985,630, 4,135,997, 4,230,545, 4,272,340, 4,460,442, 4,834,851 and in Italian patent application no. 67723A/82, it may be concluded that :
- anodes made of carbon or graphite, as such or coated by lead dioxide, are known in the art but offer a rather limited active lifetime, in the range of a hundred hours due to the oxidizing action of oxygen evolution. Obviously this brings forth higher maintenance costs for substituting the anodes and additional costs connected to the consequent production losses;
- anodes made of titanium, coated by lead dioxide or platinum or oxides of the platinum group metals, still undergo corrosion, though to a far less extent with respect to carbon or graphite, in any case insufficient for counterbalancing the higher construction costs;
- anodes made of tantalum coated by platinum metal or metal oxides offer a much longer lifetime than titanium but the production costs are extremely high;
- the parasitic reaction of lead dioxide deposition onto any type of anode may be prevented adding a suitable inhibitor to the leaching solution, for example phosphoric acid, antimony acid or arsenic acid. However, the quantities required may spoil the compactness of the lead metal deposit. This problem is overcome by resorting to an anode having a coating made of metals or oxides of the platinum group metals and at least one element comprised in the group of arsenic, antimony, bismuth, tin. In this case a remarkably lower quantity of inhibitor to prevent the anodic deposition of lead dioxide is required and the deterioration of the produced lead deposit is eliminated.
[0003] It is therefore evident that the prior art does not provide for an anode offering both a long lifetime (higher than 1000 hours) and a limited cost, which are both necessary features for a wide industrial application.
[0004] It has been surprisingly found that ceramic anodes made of sinterized powders of tin dioxide doped by suitable additives both to facilitate sinterization and to increase their electrical conductivity show an exceptional resistance to the aggressive action of acid solutions containing anionic fluorocomplexes, even under the severe conditions of oxygen evolution at high current densities (e.g. 2000 A/m²).
[0005] It has been further found that said ceramic anodes can be obtained by production techniques which are more simple and less expensive than those conventionally used to obtain ceramic products (isostatic pressing at 1200-2000 kg/cm² and sinterization at 1350-1450°C for 50-200 hours indicatively), irrespective of their functional characteristics, in particular of electrical conductivity,
Furthermore, it has been found that the oxygen evolution voltage of said anodes is considerably decreased, with the consequent advantageous decrease of the energy consumption, if the solutions containing metal ions and fluorides and/or anionic fluorocomplexes are added with suitable compounds. The same result is alternatively obtained by applying onto said anodes suitable coatings resistant to corrosion and provided with electrocatalytic activity for oxygen evolution.
[0006] Eventually, it has been found that the parasitic reaction of deposition of oxides of high valence metal ions on said anodes is efficaciously controlled by adding suitable inhibitors to the solutions containing the metal ions, fluorides and/or anionic fluorocomplexes.
[0007] The attempt to find an alternative technique to the conventional industrial production technique has been pursued with the aim to obtain, in large quantities and at low costs, products with a more complex geometry than the simple cylinder or tile so far available on the market, as for example tubes or hollow prism structures, as required for the anodes of the present invention. The technology illustrated in the following description permits to attain the aforesaid objects and eliminates the isostatic pressing step. It is characterized in that it comprises:
- precalcining the tin dioxide powder
- mixing the precalcined powder with powders of suitable additives to promote sinterization and improve electrical conductivity
- wet casting in moulds, for example in alabaster moulds
- drying in forced air
- sinterization at remarkably lower temperatures than the destabilization point of tin dioxide (1600°C) but at the same time within extremely reduced times (4-10 hours)
2CeIII - 2e⁻ -------> 2CeIV
2CeIV + H₂O -------> 2CeIII + 1/2 O₂ + 2H⁺
2 CeIII - 2e ⁻-------> 2CeIV
An alternative procedure to obtain the same result, particularly advantageous when, for process reasons, the solution cannot be added with compounds of cerium and/or praseodymium, consists in applying to the ceramic anode, made of doped tin dioxide, an electrocatalytic coating directed to favouring oxygen evolution. This coating does not comprise metal of the platinum group or compounds thereof but is made of oxides of transition elements such as the lanthanides, for example cerium or praseodymium, added with other elements to increase their resistance to corrosion and the electrical conductivity, for example niobium, nickel, copper and manganese. Alternatively this coating may be made of manganese dioxide, doped by copper and chromium.
[0008] As regards the deposition onto the anode surface of oxides of high valence metal ions, such as PbO₂, SnO₂ formed by oxidation of the metal ions present in the electrolytic solutions Pb⁺⁺, Sn⁺⁺, it must be pointed out that this side-reaction should be hindered as much as possible. In fact, the formation of oxides decreases the cathodic efficiency of metal deposition and in the long run brings to the formation of muds which make the regular operation of the electrolysis cell difficult. Technical literature describes the use of additives, such as phosphoric acid, antimonic acid, arsenic acid, which, once added to the solutions, inhibit formation of metal oxides. In order to obtain the best efficiency when used with the anodes of the present invention, these additives must be present in suitable concentrations not to spoil the quality of the metal deposited onto the cathode causing embrittlement and pulverization of the same. It has been found that zirconyl phosphate completely inhibits these negative by-side reactions. In fact this compound bars formation of metal oxides at the anode even when present in minimum concentrations. Further, it has been surprisingly found that zirconyl phosphate may be applied as an external layer onto the anodes of the invention already provided with an electrocalytic coating. This external layer can inhibit formation of high valence metal oxides so that the addition of zirconyl phosphate to the solution may be reduced to extremely low levels, thus increasing the quality of the metal obtained at the cathode.
[0009] These and other features of the present invention are illustrated in the following Examples which, however, should not be intended as a limitation of the present invention.
EXAMPLE 1
[0010] Eleven rods, having a diameter of 10 mm and a length of 100 mm, have been prepared according to the following procedure:
- precalcination of tin dioxide powder (800 - 1200°C for eight hours, average final size of the particles: 1-20 microns)
- mechanical mixing, in a ball mill, of the tin dioxide powder and additives necessary to favour sinterization, in alternative to CuO, conventionally used in the prior art;
- dispersion of the powders in an aqueous medium with the addition of nitrogen bearing surfactants;
- casting in an alabaster mould
- natural drying followed by drying at 60-120° in forced air
- sinterization at 1250°C in a gas-fired oven for 8 hours
The results reported in Table 1 lead to the following conclusions:
- all the additives exhibit a sinterizing action;
- the additives used in admixtures are characterized by a greater efficiency with respect to the same additives used alone (synergism);
- when the additives are used alone, at the same concentration and sinterization conditions
(temperature and time), the efficiency increases according to the following order:
ZnO < Nb₂O₅ < NiO < Ta₂O₅ < CuO;
- when the additives are used in admixtures and at the same sinterization conditions,
the efficiency increases according to the following order:
CuO + ZnO < CuO + Nb₂O₅ < CuO + Ta₂O₅ < CuO + NiO.
[0011] The same results have been obtained with tubes having an internal diameter and an external diameter respectively of 22 and 30 mm and a length of 120 mm produced by continuous extrusion. Apart from the extrusion procedure, the other production steps remained unvaried with respect to the above described wet casting procedure, in particular as regards temperatures and times.
EXAMPLE 2
[0012] 38 tubes having internal and external diameter of 22 and 30 mm respectively and a length of 120 mm have been prepared according to the extrusion and sinterization procedure illustrated in Example 1, utilizing composition no. 2 of Example 1, containing further additives to decrease the electrical resistivity. The density and electrical resistivity have been detected on the tubes thus obtained and the results are reported in Table 2.
[0013] The results reported in Table 2 lead to the following remarks:
- all the additives promote electrical conductivity at low temperatures;
- for each additive a threshold concentration has been defined beyond which the promoting action no more increases or even decreases;
- when the additives are used alone, the promoting action increases according to the
following order:
La2O₃ < Al₂O₃ < Cr₂O₃ < Fe₂O₃ < Bi₂O₃ < Pr₆O₁₁ < Sb₂O₃
- if used in admixtures (binary system), the promoting action is higher than that of the components used alone;
- in particular, the promoting action of the couples of additives increases according
to the following order:
Sb₂O₃ + La2O₃ < Sb₂O₃ + Al₂O₃ < Sb₂O₃ + Cr₂O₃ < Sb₂O₃ + Bi₂O₃ < Sb₂O₃ + Pr₆O₁₁ < Sb₂O₃ + Fe₂O₃
Further tests directed to decrease the electrical resistivity by keeping the composition unchanged and by modifying the sinterization temperature indicated that the temperature must be maintained in the range of 1250-1350°C, preferably 1300-1350°C.
[0014] Further tests on the efficiency of other additives, in addition to those described in this Example, showed that silver as a metal or oxide and oxides of cerium, neodimium, titanium give positive results. It may be concluded that low electrical resistivities may be obtained by adding oxides (or even metals in some cases) of elements of groups VA, IA, IIIA, IIIB, IVB, VB, VIII of the Periodic Table.
EXAMPLE 3
[0015] Emispheric caps, having a diameter of 30 mm have been produced by wet casting The composition was the same as that of the tube no. 4 of Example 2. The caps have then be welded to tubes, having internal and external diameter of 22 and 30 mm respectively, a length of 120 mm and a composition as given in Example 2, sample No. 4 using a ceramic enamel having a low melting point comprising tin dioxide added with lead oxide (0.5 - 5%), antimony, copper and cerium (for a total of 5 to 10%). The tube-cap assemblies have been sinterized at 1250°C and a current feeder has then been applied thereto, according to the following procedure:
- pretreatment of the internal surface of the tubes by corindone blasting and ultrasound cleaning
- introduction inside the tubes of a copper rod having a diameter of 18 mm
- interposition in the gap between the tube and the copper rod of a conductive filling made of copper powder suspended in an organic medium, or copper (50%) and silver (50%) powders suspended in an organic medium, or scales of Wood alloy, alloy 78 (bismuth 50%, lead 25%, tin 15%, indium 10%) or equivalents;
- evaporation of the medium or melting to the low melting alloy and subsequent cooling and solidification.
[0016] The electrical resistance of the electrical contact has then been determined, resulting in a very high value (15-1000 ohm) for all of the samples made of copper or silver-copper powders. Conversely, the resistance of the samples based on low-melting alloys was extremely lower and quite satisfactory (0.002-0.005 Ohm).
[0017] The same results have been obtained substituting the copper rod with copper wires or copper strands.
[0018] Likewise satisfactory results have been obtained with the electrical contacts based on low melting alloys, which remain liquid even at the operating temperatures of electrolysis when the samples have been used as anodes. Suitable alloys comprise lead (24%), tin (14%), indium (10%), gallium (2%), bismuth (50%).
EXAMPLE 4
[0019] Some tubes, provided with the emispheric caps and current feeders have been prepared as described in Example 3 and used as anodes at the following conditions:
The samples, made of tin dioxide containing 1% copper oxide and 2.5% antimony oxide, as already illustrated in Example 3, had been previously sandblasted on the internal surfaces by corindone. The electrolytic solutions were used as such or added with inhibitors of the anodic formation of lead dioxide. Phosphoric acid, known in the art, and zirconyl phosphate were utilized as inhibitors. The solutions containing 2000 ppm of zirconyl phosphate were further added with compounds capable of acting under homogenous phase as catalysts for the oxygen evolution reaction. In particular, compounds capable of releasing into the solutions the ionic couples CeIII/CeIV and PrIII/PrIV were selected. The results of the tests expressed as anodic voltages, lead dioxide formation as the parasitic reaction and quality of the plated lead are reported in Table 4. The concentrations of the additives in the solutions are expressed as ppm (parts per million)
No appreciable corrosion of the anodes was observed. The data reported on table 4 clearly show that the anodes made of the tubes and caps are compatible with the electrolysis process in solutions containing fluorides and anionic fluorocomplexes as regards the composition, the mechanical stability and the type of electrical contact. The anodic voltages are stable with time and may be further decreased to interesting values for industrial applications by adding to the solutions suitable compounds to catalyze the oxygen evolution reaction. Furthermore, the parasitic reaction of lead dioxide formation, as well as similar parasitic reactions which could take place with different metal ions, is efficiencly prevented by adding to the solutions zirconyl phosphate. This additive, never disclosed in the prior art, requires low concentrations (e.g. 2000 ppm) not to impair the quality of the metal plated to the cathode.
EXAMPLE 5
[0020] Tubes provided with caps as described in Example 3, made of tin dioxide added with copper oxide (1%) and antimony oxide (2.5%) were sandblasted with corindone on the internal surface and coated by a a coating based on oxides of cerium, praseodimium, manganese, as such or in combinations thereof, further doped by oxides of the elements of the group of niobium, copper, nickel and chromium.
[0021] The coating was directed to catalyze the oxygen evolution reaction avoiding the need to add elements as described in Example 4. The coatings were obtained by applying paints containing precursors salts such as resinates, subsequently thermally decomposed in air at 1250°C, as known in the art, as taught for example in U.S. Patent No. 3,778,307.
[0022] Alternatively, said coatings are obtained by applying paints based on suspensions of preformed powders of the aforementioned oxides, said powders having an average diameter in the range of some microns and the suspensions being stabilized by nitrogen bearing surfactants. The paints were then applied by brush or spray, followed by thermal treatment in air at 1250°C for three hours. In both cases, the cycle painting-thermal treatment is repeated until a thickness of the coating of about 100 microns is obtained.
[0023] The various samples were tested as anodes in the following solutions and at the following conditions:
The samples were then characterized as follows:
| No.1 CeO₂ | paint with precursors |
| No.2 CeO₂ + Nb₂O₅(5%) | paint with precursors |
| No.3 CeO₂ + Nb₂O₅(5%) | paint as suspension |
| No.4 CeO₂ + Nb₂O₅(5%) + NiO(2%) | paint with precursors |
| No.5 CeO₂ + Nb₂O₅(5%) + NiO(2%) | paint as suspension |
| No.6 CeO₂ + Nb₂O₅(5%) + CuO(2%) | paint with precursors |
| No.7 CeO₂ + Nb₂O₅(5%) + CuO(2%) | paint as suspension |
| No.8 CeO₂+Nb₂O₅(5%)+NiO(2%)+CuO(1%) | paint with precursors |
| No.9 Pr₆O₁₁ | paint with precursors |
| No.10 Pr₆O₁₁ + Nb₂O₅(5%) | paint with precursors |
| No.11 Pr₆O₁₁ + Nb₂O₅(5%) | paint as suspension |
| No.12 Pr₆O₁₁+ Nb₂O₅(5%) + CuO(2%) | paint with precursors |
| No.13 Pr₆O₁₁+ Nb₂O₅(5%) + CuO(2%) | paint as suspension |
| No.14 CeO₂ + Nb₂O₅(5%) + CuO(2%) + + Pr₆O₁₁ (2%) | paint with precursors |
| No.15 CeO₂ + Nb₂O₅(5%) + CuO(2%) + + MnO₂ (2%) | paint with precursors |
| No.16 MnO₂ | paint with precursors |
| No.17 MnO₂ + CuO(2%) + Cr₂O₃(2%) | paint with precursors |
The experimental data are collected in Table No. 5.
[0024] No formation of lead dioxide was experienced. The data reported on Table 5 clearly show that the tubes made of tin dioxide added with copper and antimony oxide may be provided with a coating having a strong resistance to the aggressive attack of the electrocatalytic solutions and concurrently having a remarkable electrocatalytic activity for the oxygen evolution reaction. Similar results have been obtained using these samples in a similar solution as the one used to obtain the data reported in Table 5, the only difference being the addition of fluorosilic acid (120-140 g/l) instead of fluoroboric acid.
EXAMPLE 6
[0025] Five anodes prepared as sample no. 6 of Example 5 were further coated with a zirconyl phosphate layer, obtaining a thickness varying from 10 to 250 microns, by plasma spray technique. The samples were used as anodes at the same conditions as illustrated in the previous examples, the only difference being that no inhibitors were added to avoid formation of lead dioxide. The tests showed that with layers of zirconyl phosphate above 50 micron, no lead dioxide formation is experienced. However said thickness must be maintained below 250 micron to avoid increasing the anodic voltage.
1. Sinterized ceramic anode for oxygen evolution comprising tin dioxide and additives
to promote sinterization and increase electrical conductivity characterized in that
said additives are metals or metal oxides selected in the group comprising elements
of the Groups IB, IIB, IIIA, IIIB, IVB, VA, VB, VIB and VIII of the Periodic Table,
as such or in admixtures thereof, in concentrations by weight comprised between 0.5
and 5%.
2. The anode of claim 1 characterized in that said additives to promote sinterization
comprise 1% by weight of copper oxide and 1.5% by weight of nickel oxide.
3. The anode of claim 1 characterized in that said additives to increase electrical conductivity
comprise 2.5% by weight of antimony trioxide and 1% by weight of ferric oxide.
4. The anode of claim 1 characterized in that said anode further comprises an external
electrocatalytic coating for oxygen evolution made of manganese dioxide as such or
at least one oxide selected from cerium dioxide, praseodymium oxide, manganese dioxide
further mixed with at least one additional oxide belonging to the group of niobium
pentoxide, copper oxide, nickel oxide, chromium oxide.
5. The anode of claim 4 characterized in that said external coating comprises up to 9%
by weight of said additional oxide.
6. The anode of claim 4 characterized in that said electrocatalytic coating is further
coated by an external layer of zirconyl phosphate having a thickness comprised between
50 and 250 microns.
7. A process for the electrolytic deposition of metal from electrolytic solutions containing
fluorides or fluorocomplex anions, carried out in an electrochemical cell provided
with at least one anode and one cathode, characterized in that said anode is of the
type described in claims 1, 2 and 3 and the electrolytic solution contains catalysts
for oxygen evolution selected from compounds of cerium and/or praseodymium.
8. The process of claim 7 characterized in that the concentration of said catalysts is
higher than 1000 ppm.
9. A process for the electrolytic deposition of metals from electrolytic solutions containing
fluorides or fluorocomplex anions carried out in an electrochemical cell provided
with at least one anode and one cathode characterized in that said anode is of the
type described in claims 4 and 5.
10. The process of claims 7 and 9 characterized in that said electrolytic solution further
comprises inhibitors of the anodic formation of metal oxides, said inhibitors being
selected between phosphoric acid and zirconyl phosphate.
11. The process of claim 10 characterized in that the concentration of phosphoric acid
is higher than 3000 ppm.
12. The process of claim 10 characterized in that the concentration of zirconyl phosphate
is higher than 2000 ppm.
13. A process for the electrolytic deposition of metals from electrolytic solutions containing
fluorides or fluorocomplex anions carried out in an electrochemical cell provided
with at least one anode and one cathode characterized in that said anode is the anode
described in claim 6.
14. The method for producing the anode of claim 1 characterized in that it comprises the
following steps:
- precalcination of the tin dioxide powder at 800-1200°C
- mechanical mixing with additives to promote sinterization and to increase electrical conductivity
- suspension in water of the powder mixture by means of nitrogen bearing surfactants
- casting in alabaster moulds or continuous extrusion
- natural drying and subsequent drying at 60-120°C in forced air
- bonding of accessory components with a ceramic enamel
- sintering at 1250-1350°C.
15. The method of claim 14 characterized in that said ceramic enamel comprises tin dioxide
added with lead oxide in concentrations of 0.5-5% and antimony trioxide, copper oxide,
or cerium oxide as such or in a combination of the same, in a total concentration
of 5 to 10%.
16. The method of claim 14 characterized in that it comprises producing the anode in the
form of tubes or hollow prisms provided with a current feeder according to the following
steps:
a) blasting the internal surface
b) introducing a copper rod, wires or a thread in the cavity of the anode
c) filling the space between the anode and the current feeder with a conductive filler comprising a low temperature melting alloy based on elements selected from the group of lead, bismuth, tin, indium.
17. The method of claim 14 characterized in that it comprises further coating the anode
with the coating of claims 4 and 6 according to the following procedure:
a) application of a paint containing the precursor compounds of the coating or a paint consisting in a dispersion of preformed powders of the components of the coating and a nitrogen-bearing surfactant
b) thermal treatment in air
c) repeating the above procedure up to obtaining the desired thickness.